The present invention relates to field effect semiconductor devices, and, more particularly, to the source contact of high frequency field effect devices.
Field effect transistors (FET) fabricated from gallium arsenide and with Schottky barrier gates are commonly used for microwave devices. Indeed, GaAs monolithic microwave integrated circuits (MMIC) of such devices covering a wide range of microwave applications have been reported. See, generally, the December 1983 special issues of IEEE Trans.Elec.Devices and IEEE Trans. Micwv.Th.Tech.
The typical such FET is fabricated on semi-insulating GaAs with a n-type epilayer about 0.1 to 0.4 microns thick by evaporation of aluminum gates and deposition, lift off, and alloying of gold-germanium source and drain ohmic contacts. Channel widths may be several hundred microns, whereas channel lengths are on the order of one micron. For MMICs the use of localized implantation for isolation is common and fabrication may be by the following steps: patterned photoresist and donor implant to define the active region (which will include the source, drain, and channel regions), activation of the implant, patterned photoresist and heavy donor implant to define the source and drain within the active region, activation, deposition of aluminum by vacuum evaporation, etching of the Al to define the Schottky barrier gates, and lifting off a AuGe-Ni film and alloying it at 400 degrees C. to form ohmic contacts with the source and drain regions. In either case the use of AuGe type ohmic contacts is usual. See, for example, Sugiura et al, 12-GHz-Band Low-Noise GaAs Monolithic Amplifiers, 30 IEEE Trans. Elec.Devices 1861 (1983).
For microwave operation the FET is normally biased in the saturation region, and in the saturation region the transconductance is strongly affected by the parasitic source resistance (the resistance primarily between the active channel and the ohmic source contact). See, S.Sze, Physics of Semiconductor Devices p.341 (2d Ed. 1980), where the measured transconductance of a real FET is equal to gm/1+g.sub.m R.sub.s with gm being the ideal transconductance and R.sub.s the source resistance. Further, the source resistance also degrades the noise figure and power performance of the FET. Consequently, efforts to reduce the source resistance have been made and include moving the source contact closer to the active channel, recessing the gate, and increasing the doping underneath the source contact. See, Sugiura, cited above, commenting that the recessed gate degrades the uniformity of the active layer and using shortened source to gate spacings appears a better approach. However, M. Heilblum et al, Characteristics of AuGeNi Ohmic Contacts to GaAs, 25 Solid-State Elec.185 (1982), suggest that the formation of the usual ohmic contacts for the FET leads to limitations on the ability to move the source contact closer to the active channel and reduce the source resistance; namely, a high resistance layer under the contact several thousand angstroms deep dominates the contact resistance, and a peripheral zone extending about one micron from the contact into the GaAs exists in which the GaAs chemically differs from the rest of the GaAs. Source resistances in the range of three to four ohms for a three hundred micron wide gate seem to be the best obtainable with the known FET structures.
D.Meignant and D.Boccon-Gibod, Schottky Drain Microwave GaAs Field Effect Transistors, 17 Elec.Lett. 107 (1981) have suggested using a Schottky barrier drain contact instead of an ohmic contact, but note that ohmic source contact (gold-germanium) is still a necessity.
Thus the known FET structures have a problem of high source resistance which degrades device performance.